The author has no FORMAL contract, but there is certainly an expectation that a series will be finished at some point. It is certainly the case that when someon

The author has no FORMAL contract, but there is certainly an expectation that a series will be finished at some point. It is certainly the case that when someone purchases one of a series of works, unless it is of a nature that it's expected to continue indefinitely (E.G. any given Marvel comic for example) people generally expect that at some point in the future the series will conclude. At a bare minimum they expect the current story arc to be resolved in something approaching a satisfactory way (for varying values of satisfactory). Importantly there is no guarantee of when precisely that conclusion will be provided, or truly a guarantee it will be provided at all, but there is certainly the expectation. Further once an author has set a known "pace" to his writing people tend to expect him to continue that pace (rightly so or not), so when he massively deviates from that pace people tend to take notice. E.G. if an author were to publish 3 books in a series, one every year, and then suddenly go 4 or 5 years before publishing the next book in the series people would most likely be very angry with that author (wrongly or not, it's more a question of human nature). So no, while the author is not in fact their "bitch" as Neil Gaiman puts it, there is a certain expectation on both sides, namely the audience expects the work to be concluded at some point in the future and for work to be released at something approaching a regular schedule, while the author should then also have the reasonable expectation that those purchasing his books will continue to do so in the future. Generally both parties end up with their expectations largely met, and things work out for everyone.